Conventionally, a coated cutting tool having a vapor-deposited hard coating layer is known. The conventionally-known coated cutting tool has a body (hereinafter referred to as a cutting tool body) made of tungsten carbide-based cemented carbide (hereinafter referred to as a WC) or titanium carbonitride-based cermet (hereinafter referred to as a TiCN). It also has a vapor-deposited hard coating layer formed on the cutting tool body. The hard coating layer is constituted of (a) a lower layer that is a Ti compound layer made of one or more layers selected from a Ti carbide (hereinafter referred to as a TiC) layer, a Ti nitride (hereinafter referred to as a TiN) layer, a Ti carbonitride (hereinafter referred to as a TiCN) layer, a Ti carboxide (TiCO) layer, and Ti oxycarbonitride (TiCNO) layer, and (b) an upper layer that is an alumina layer (hereinafter referred to as a Al2O3 layer) having an α-type crystal structure in a chemically-deposited state.
The above-mentioned conventionally-known coated cutting tool exhibits excellent abrasion resistance on a variety of steel or casted iron in a continuous cutting or in an intermittent cutting. However, when it is subjected to a high-speed heavy cutting or a high-speed intermittent cutting, flaking or chipping of the coating layer is likely to occur, shortening its tool life.
Under such circumstances, in order to suppress flaking and chipping of the coating layer, various coated cutting tools with improved lower and upper layers have been proposed.
For example, Patent Literature (PTLs) 1 and 2 are known as ways to improve the lower layer. In the lower layer of the coated cutting tool described in PTL 1, the grain width of the TiCN layer of the lower layer is reduced, and the surface roughness on the surface of the hard coating layer is set to an appropriate value in order to improve its impact resistance, fracture resistance, and abrasion resistance. In the lower layer of the coated cutting tool described in PTL 2, a TiCNO layer having thickness of 2 to 18 μm is formed at least as the Ti compound layer. In the TiCNO layer, the surface with the strongest X-ray diffraction peak is a (422) surface or a (311) surface. In addition, the oxygen content in the TiCNO layer is 0.05 to 3.02 mass %. In addition to the above-described configurations, the width of TiCN crystal grains is reduced in the coated cutting tool described in PTL 2. Accordingly, coarsening of crystal grains on the surface of the hard coating layer and formation of local protrusions are intended to be prevented. Moreover, improvements in the strength of the TiCNO layer itself and adhesion between the lower and upper layers are attempted.
For example, PTLs 3 and 4 are known as ways to improve the upper layer. In the coated cutting tool described in PTL 3, improvements of abrasion resistance and fracture resistance are attempted by configuring the peak intensity of a (030) surface, I (030), in X-ray diffraction is stronger than the peak intensity of a (104) surface, I (104), in the Al2O3 layer of the upper layer. In the coated cutting tool described in PTL 4, the Al2O3 layer consisting of the upper layer is configured to be a dual-layer structure made of a top and bottom layers. Further, when a inclination distribution graphs are drawn in the range of 0 to 45° in the case of the top layer and in the range of 45 to 900 in the case of the bottom layer by measuring inclinations of the normal line of a (0001) surface using an electron field emitting scanning electron microscope, the highest peak exists in the inclination classification in the range of 0 to 15° and the sum of frequency in the range corresponds to 50% or more of the total in the top layer, and the highest peak exists in the inclination classification in the range of 75 to 90° and the sum of frequency in the range corresponds to 50% or more of the total in the bottom layer. By having the dual-layer structure configured as described above, chipping resistance is improved in the coated cutting tool described in PTL 4.